1. Field of the Invention
The present invention relates to a composite magnetic material with reduced permeability and losses at frequencies below about 100 MHz.
The material is designed especially for making inductor cores or transformer cores.
2. Description of the Prior Art
In the development of electronic systems, it is sought to miniaturize the supply sources. The change from linear structure regulators to switched-supply converters is a decisive step in reducing the amount of space used and in improving the performance characteristics of the supply sources. The switching frequency has been constantly increasing with a view to further miniaturization. Present-day converters attain and even exceed frequencies of one MHz. Architectures using low value inductance (in the range of some micro-Henrys) are likely to have low total losses (conductor and magnetic circuit losses) under high induction and low permeability (below 200 approximately).
Magnetic materials with reduced permeability currently available on the market have very high losses under high induction (of over 10 mT). This means that, today, magnetic components are the bulkiest part of the converters. For existing magnetic materials, the low permeability and low losses at high frequency are contradictory characteristics.
An inductor with an inductance value of some micro-Henrys will have a few turns or a core with low permeability.
A small number of turns taken to a high potential difference generates high magnetic induction in the core. Since the losses in the core are at least proportional to the square of the induction, they grow very rapidly when the number of turns decreases. To obtain smaller losses, it is necessary to have a large number of turns. This requires a core with low permeability.
There are air-based inductors with non-magnetic cores. Their permeability is equal to one and the losses in the core are zero. Their size is great because of the permeability of the non-magnetic core which is equal to one. The "copper" losses dissipated by the coil are great. The electromagnetic disturbances generated are troublesome for the vicinity and difficult to eliminate.
There are magnetic core inductors made of localized air gap spinel-type massive ferrite. The ferrite, despite its losses in the range of one-hundredth or one-tenth W/cm.sup.3, depending on the induction and the frequency, has permeability values in the region of 1000. This is far too high for the application of the converters. Ferrites with low permeability such as nickel ferrite which have permeability of 10 have excessively high losses for the application of the converters.
There also exist inductors with distributed gap composite magnetic cores. These materials are formed by ferromagnetic alloys made of powder dispersed in a dielectric binder. The losses by radiation are smaller than those of localized gap cores. There are essentially two categories of powders: powdered iron and carbonyl iron powder whose permeability ranges from 5 to 250 approximately and powders based on iron-nickel alloys whose permeability ranges from 14 to 550 approximately.
The losses in these materials are fifteen to twenty times greater than those of massive power ferrites under the same conditions of frequency, induction and temperature.
For example, the best composite magnetic materials on the market have the following characteristics (according to data from the supplier's catalog) for toroidal or ring-shaped samples having an average diameter of 10 mm, at ambient temperature, for an induction value of 30 mT at 1 MHz:
carbonyl iron: losses of over 1.5 W/cm.sup.3 PA1 nickel iron: losses of over 2 W/cm.sup.3. PA1 the making of a ceramic magnetic powder; PA1 the making, from the ceramic magnetic powder, of a casting slip; PA1 the cutting out of the wafers from a film of the casting slip; PA1 the sintering of the wafers; PA1 the preparing of the composite magnetic material from the sintered wafers, dispersed in the binder, the main faces of which are oriented with respect to the magnetic field.